Magellan resumes mission to map Venus.

Thanks to a flawed but still functioning backup transmitter, the Magellan spacecraft was set to resume its radar mapping of Venus on Jan 24, nearly three weeks after the main transmitter failed. A spurious electronic "whistle" in the backup unit, however, will reduce by 43 percent the rate at which Magellan beams its radar data back to Earth. This will force the craft to image smaller swatches of the Venusian surface, says Magellan project engineer David Okerson of Space Applications International Corp. in Washington, D.C.

Problems with the main transmitter began Jan. 4, when the craft attempted to send its newest data to a radio receiver on Earth. Although it broadcast a clear carrier frequency -- the electronic signal that normally carries the mapping data -- for unknown reasons the transmitter stopped adding the Venus data to the monotone carrier signal. In effect, the transmitter beamed to Earth a single note of music rather than the symphony of information describing Venus' surface.

Since the craft has a limited capacity to store radar data, the transmission failure forced NASA scientists to halt radar studies until Magellan resumed full communication with Earth. (Despite the equipment problem, the craft could still receive electronic signal critical to maintaining its proper orbit around Venus, Okerson notes.)

Mapping came to a halt during Magellan's 3,880th orbit around Venus. Since September 1990, the craft's radar has pierced the clouds enshrouding Venus to image 95 percent of the planet's surface -- a rugged terrain marked by volcanoes, craters and sinuous channels (SN: 12/21&28/91, p.424).

With the main transmitter out of commission, switching to the backup would normally entail a relatively simple series of operations, Okerson observes. But the backup unit had its own problems: Engineers had turned it off in March after electronic noise, known as a whistle, impaired its ability to send clear signals to Earth. The electronic frequency of the whistle drifts and its intensity increases as the transmitter's temperature rises. Researchers believe a crack in a computer chip may account for the whistle, since such a fracture would shrink or expand with temperature changes.

Earlier this month, NASA engineers managed to circumvent most of the wistle by choosing a lower-than-normal electronic frequency--well-separated from the obscuring noise -- for the radar data, Okerson says. They also decided to leave the backup transmitter turned on instead of powering it up only when sending information, so that its temperature--and, they hoped, the whistle -- would remain constant.

At the lower frequency, notes Okerson, the backup transmitter relays data at a rate of 115 kilobits per second -- less than half the previous rate. This slowdown decreases the area Magellan can map during each orbit.

Because the other part of Magellan's mission -- probing the planet's interior by measuring its surface gravity -- depends only on the standard carrier signal, the transmitter's slower relay of radar data does not affect it, Okerson says.